2.0 Analysis 2.1 Introduction The investigation into this accident included examination of environmental, technical, human, and operational factors, and an in-depth examination of the helicopter and its component parts, including their service life and history. There was nothing found during the investigation indicating that the total weight of the helicopter or the external load played any part in this occurrence. The practices and procedures used in the logging operation were evaluated, and no remarkable deviations were noted. Extensive examination and testing of the helicopter and its systems revealed that the only anomaly was in the yaw axis of the aircraft flight controls, specifically the U12 analog switch in the yaw axis of the No.2 AFCS computer. 2.2 Flight Control System Malfunction The position of the yaw LBA piston when the cylinder burst was in a right yaw condition. Since the hydraulic condition to burst the cylinder requires the servo valve be in a closed or partially open position, it can be said that the LBA piston was in a stabilized right yaw position immediately before the burst occurred. Such a condition is likely to have been the result of a normal yaw input by the pilot to begin the helicopter turning right. It is likely that the U12 switch failed at this point and began the chain of events that caused the LBA to burst. The investigation determined that when the U12 analog switch in the yaw axis of the AFCS failed, 11 VDC was applied directly to the No.2 yaw ELA. As a result, the No.2 yaw ELA rapidly and fully extended. It could not be determined why the cross-coupling function of the AFCS did not counter the erroneous signal, but it is possible that the rate of this signal exceeded the cross-coupling capability of the No.1 AFCS. Under normal circumstances, a controlled extension force from the ELA is transmitted to the control mixing complex, which then translates the extension into mechanical inputs to the UBAs. In the accident, a sudden force displaced the input rod seal in the No.1 LBA and caused the No.2 LBA cylinder wall to burst. The downward motion of the LBA piston also caused the yaw connecting link to buckle and break under column-loading. It is likely that the sudden force was the result of rapid ELA extension with the ELA output meeting a control system resistance, causing the force to instantly rebound down to the yaw LBA output rod. The control system resistance may have occurred because the extension speed exceeded the capability of the UBAs to respond. It is also possible that the sudden fracture of the yaw connecting link created an upward input to the LBA servo valve and amplified the right turn. Hydraulic control to the other axes of flight was available, as demonstrated by the apparent lateral and vertical control during the rotation leading to impact, and by the indications of system pressure captured on the hydraulic pressure gauges in the cockpit. It is clear from the yaw pedal damage and injury to the pilots that they were attempting to counter the right yaw when the helicopter crashed. Had the connecting link still been intact, any left pedal input would have been transmitted to the LBA and even with the burst cylinder, flight control movement would have been transmitted to the rotor systems with effect. Furthermore, full left pedal would have caused the servo valve to remain open, thereby removing one of the conditions for bursting the LBA cylinder. However, the helicopter did not respond to the pilots' corrective action because the broken yaw link prevented their left yaw input from reaching the LBA. Under the circumstances of the rapid, right yaw, the pilots would likely have become disoriented and unable to prevent the helicopter from striking the terrain. 2.3 Survival Factors The degree of destruction as a result of impact forces made this accident unsurvivable. It is known that the pilots were not wearing their shoulder harnesses at the time of impact. Although it is doubtful that the use of the shoulder harness would have lessened either pilot's injuries, the benefits of the torso restraint were greatly compromised, which likely reduced either pilot's ability to effectively control the helicopter. Accident investigation and research carried out by the TSB has consistently shown that the use of the shoulder harness portion of the seat restraint system is effective in reducing or preventing injury during moderate impact forces. Given that vertical reference flying requires upper-body freedom of movement, the practice of not using the shoulder harness will continue to be widespread.The risk associated with this practice is that pilots will not be restrained effectively in the event of an in-flight emergency. 3.0 Conclusions 3.1 Findings as to Causes and Contributing Factors The U12 analog switch installed in the yaw axis of the No.2 AFCS computer failed in electrical overload, and sent an instantaneous extension signal to the No.2 yaw ELA. The rapid ELA extension in the yaw flight control system almost certainly caused the yaw LBA to burst and broke the yaw connecting link, preventing the pilot from countering a right-yaw condition. Without yaw control, the pilots likely became disoriented and could not prevent the helicopter from striking the terrain. 3.2 Findings as to Risk The practice of not using shoulder harnesses during vertical reference flying exposes pilots to greater risk of ineffective restraint during an in-flight emergency. 3.3 Other Findings No indication was found of any malfunction or pre-existing mechanical defect with the engines or related systems that could have contributed to the accident. The only pre-impact anomalies identified with the helicopter during the investigation were the U12 analog switch in the yaw axis of the flight control system, the burst yaw LBA, and the fractured yaw connecting link. No other systems revealed pre-impact failures. Columbia Helicopters Inc. determined, with the agreement of the Boeing Company, that the likelihood of a recurrence of the identified malfunction falls outside the certification limits (a probability of failure of 10-9). Work to reduce further the likelihood of this type of failure is, therefore, not required. However, Columbia Helicopters Inc. has undertaken intensive experimentation and research in an effort to identify any further causal elements.4.0 Safety Action Columbia Helicopters Inc. determined, with the agreement of the Boeing Company, that the likelihood of a recurrence of the identified malfunction falls outside the certification limits (a probability of failure of 10-9). Work to reduce further the likelihood of this type of failure is, therefore, not required. However, Columbia Helicopters Inc. has undertaken intensive experimentation and research in an effort to identify any further causal elements.